(09-01) 17:42 PDT Martinez -- All lanes on southbound Interstate Highway 680 have reopened after a big rig carrying cars caught fire in Martinez late Monday afternoon, forcing the closure of all but one lane, according to the California Highway Patrol.

The big rig fire, first reported at 2:10 p.m., sparked a blaze on a hillside near the site of the incident on southbound 680 near Pacheco Boulevard.

All lanes except the far left southbound lane were closed, but they reopened at 5:14 p.m., said CHP Officer Damian Cistaro.

The lane closures, which continued after the fire had been extinguished in order to tow the big rig, had prompted a backup of traffic from the long Labor Day holiday that extended to the Benicia Bridge.

The cause of the fire remains unknown, said CHP Officer Kevin Bartlett. He said condition of the driver is unclear, but Bartlett said he did not receive any reports of injuries.